Conventionally, there has been used an image forming device provided with a sheet transporting device which can carry out reversal transport so as to reverse and transport a sheet (recording sheet, transcription sheet). Such reversal transport is carried out to stack sheets, on which images have been formed, onto a discharge tray in a desired order (in an order based on a page number) in case of carrying out double-side printing by which images are formed on both surfaces of each sheet or in case of forming images on a plurality of sheets.
For example, in case of carrying out the double-side printing, an image is formed on a front surface of a sheet, and then the sheet is reversed by the reversal transport, so as to form an image on a rear surface of the sheet.
Further, depending on a position in which a sheet having an image is discharged, the sheet may be discharged with its printed surface facing upward when the reversal transport is not carried out. In the case where the sheet is discharged with its printed surface facing upward, there is raised the following problem: when images are sequentially printed on a plurality of sheets, a page order in which discharged sheets are stacked on the discharge tray is inversed. Then, the reversal transport of sheets is carried out, so that it is possible to stack the printed sheets on the discharge tray or the like in a desired page order even in case where images are sequentially printed on the plurality of sheets.
However, such reversal transport has a more complicated mechanism than that of normal single-direction transport, so that this raises such problem that transport failure tends to occur depending on a kind of a sheet used. For example, in case of carrying out the reversal transport of tab sheets each of which has an auriform protruding portion (tab) on its end as shown in FIG. 12, the transport failure is more likely occur than in case of carrying out the reversal transport of normal rectangular sheets.
On the other hand, for example, Patent Document 1 (Japanese Unexamined Patent Publication No. 19253/2001 (Tokukai 2001-19253)(Publication date: Jan. 23, 2001) discloses a technique which relates to a sheet reversing device for carrying out the reversal transport of tab sheets while preventing the transport failure.
The sheet reversing device recited in Patent Document 1 is arranged so that: a movable reversing branch click presses a sheet to be reversed against a carrier roller, and an inverse driving roller inverses a direction in which the sheet having passed through the reversing branch click is carried, thereby carrying out the reversal transport. In case of transporting a tab sheet, a switch back timing at which a transporting direction of the tab sheet is reversed by switching a rotating direction of the inverse driving roller is delayed from a switch back timing of a normal sheet by not less than a time taken for a tab portion to pass through a sheet detection sensor (a time taken to transport a tab portion), thereby preventing transport troubles.
However, the technique disclosed by the Patent Document 1 raises such problem that: when the tab portion is detected by the sheet detection sensor, an extra time taken to transport the sheet at a distance corresponding to a tab width (tab's length in a transporting direction of the sheet) is required.
In the sheet reversing device of Patent Document 1, it is assumed that a distance from a detection position of the sheet detection sensor to the reversing branch click is K1 and a transport speed of the sheet is V1. In case of a sheet other than the tab sheet, the switch back is commenced in a predetermined time expressed by t=K1/V1 after the sheet detection sensor has detected a back end of the sheet. In case of the tab sheet, the timing at which the switch back is commenced is delayed by not less than a time, in which the tab portion is transported, in addition to the predetermined time t. That is, when the tab's length in a transporting direction is L1, the switch back is commenced in a time expressed by t+L1/V1 after the sheet detection sensor has detected a back end of the tab sheet.
Thus, there is no problem in case where the tab portion does not pass through a detection position of the sheet detection sensor. However, in case where the tab portion passes through the detection position of the sheet detection sensor, the tab sheet is excessively transported by a distance corresponding to L1/V1 or more. That is, in case where the tab portion is detected by the sheet detection sensor, the back end of the tab portion is recognized as a back end of the sheet, so that the timing at which the switch back is commenced is excessively delayed by a time taken to transport the sheet at a distance corresponding to the tab width. This lowers a printing performance (performance in an image formation process).
Further, a problem caused by uncertainty in sheet detection due to unevenness of the tab position occurs not only in case where the tab sheet is switched back. For example, in case of controlling a sheet transport interval, at which a plurality of sheets are sequentially transported, in accordance with a result obtained by detecting the back end of the sheet with the sheet detection sensor, when the tab portion passes through the detection position of the sheet detection sensor, the sheet transport interval is excessively large so as to correspond to the tab width.
In order to prevent the uncertainty in sheet detection due to unevenness of the tab position, a plurality of sheet detection sensors may be provided so as to be orthogonal to the sheet transporting direction for example, thereby always detecting portions other than the tab portion as the back end of the tab sheet. However, in this case, a plurality of sheet detection sensors are provided on respective necessary points in a transport path in a vertical direction with respect to the sheet transporting direction. Such arrangement results in higher cost.